AR234 70th anniversary

70 years ago today, on the 2nd of August 1944, the Arado Ar234 "A" flew the worlds first jet reconnaissance sortie. Piloted by Obit. Erich Sommer, it over flew the Normandy beach heads taking photos of allied activity, achieving more in one flight than the luftwaffe had been able to do in months. With fellow pilot Horst Gotz, they flew another 7 missions without detection, and followed this with flights over the UK, including London. Too fast for allied interception, the V5 of Gotz ironically fell victim to German AA fire, and crash landed, only to be subsequently rammed by a Fw 190!! The V7 of Sommers was retired from service on October 20th, after an inexperienced pilot damaged it on take off. The 'B" model was ready for service by this stage, and took over in the bomber /recon role, and that in itself is another chapter. Overshadowed by the Me 262, the Arado Ar 234 'a' achieved a number of firsts, and deserves more recognition than it got. As one documentary stated, the design of the Ar 234 was found to be so correct, its prototypes were made operational.

Like the look of the 4 engine version, the one with the two engines podded together.
Were they test flown?

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As far as I Know, one prototype with 4 separate engines flew only once, when all BMW turbojets failed minutes into the flight. It landed relatively intact, but never flew again. The coupled version was still being developed when they ran out of time, but I'll have to go and look up my reference books to verify

Didn't Hitler prefer to see the Me262 used as a bomber rather than a fighter?
Maybe he anticipated the benefits of the Ar234, only the plane was a bit late in coming.

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Hitler knew he needed something to break through the allied fighter screens to attack and destroy the up coming allied invasion. He was assured that the Me 262 could be used as a bomber and his interest and subsequent 'backing' of the aircraft in fact had a positive effect that if anything accelerated its development. I would say this level of detail was entirely appropriate for a commander in chief.

When the invasion came Hitler asked about the Me 262, which had been undergoing operational tests, and was told that precisely non had been prepared for bombing. Not only that, he was now being told about how difficult it was to prepare the aircraft for carrying bombs, something which was now the opposite of what he had been told.

He noted, presumably with some sarcasm or bitterness, that precisely non of his instructions had been listened too. He angrily forbade any Me 262 from being produced that were not bombers and would not speak of the matter after that. Prior to that he had not had any objection to the aircraft being used as a fighter but he saw it primarily as a "Blitz bomber" able to attack the invasion fleet and return Luftwaffe bombers over Britain, after all the RAF were bombing all over Germany while the massive resources needed to successfully operate conventional aircraft over British airspace were tied up in the Soviet Union. He apparently had felt that conventional aircraft could deal with the RAF and USAAF bombers back in 42/43 when he had put his support behind the Me 262.

This all had precisely no effect on the deployment of the Me 262 since the Jumo 004 engine and not the airframe was still the bottle neck, Hitler's interest had in fact accelerated development of the aircraft. I believe Erhardt Milch lost his job over the issue.

Eventually a universally liked Luftwaffe Officer noted for his judicious charm was able to persuaded him to allow 50% production as fighters so long as the fighters could be converted to fighter bombers. Face saving accomplished, and a point having been made, development preceded along sensible lines.

Post war people like Adolf Galland, the Luftwaffe's Chief Fighter pilot (noted for his fights with Goering) told a story in which Hitler and Luftwaffe brass were responsible for delaying the aircraft while he, promoted it as a fighter, a solution to the allied bombers. While Galland was most certainly responsible for advancing the Me 262 as a fighter. The story about the delays can't really be true, Hitler was dead, obviously not around to explain his thinking and it also helped to distance oneself from the nazi regime by emphasising ones disagreements.

It had been originally proposed to use the Me 262 to attack US escort fighters thus leaving conventional piston aircraft free to destroy the US bombers (something the Fw 190 did very well). Me 262 claimed quite a few US fighters and were quite willing to take them on. When the Me 262 began to become fully operational in the final months of the war it was the only thing around and it began to be used to attack the bombers themselves rather than the escorts.

Now, about the engines: the Jumo 004.

Several Ar 234A, straight from the Luftwaffe's Rechlin test centre undertook about 7 reconnaissance missions each of the Normandy Beachhead and the Cherbourg peninsula. Their 'craftsman' built Jumo 004B0 suffered not a single engine failure during these missions.

The subsequent Me 262 received the mass production Jumo 004B1 whose quality dropped due to the production quality control system not being debugged and material substitutions caused by shortages and unauthorised substitutions. This engine had solid turbine blades cold forged out of Tinidur, an iron/chrome/nickel/titanium alloy. From October/November 1944 the Jumo 004B4 (some say this was the Jumo 004B3) entered service with hollow air-cooled tinidur blades cold drawn by 10 stamping operations out of tinidur sheet. This version was more reliable and got close to its official 25 hour mean time between overhauls. From Feb 1945 some Jumo 004B4 received hollow cromadur blades (which replaced the scarce nickel with manganese) which were simply seem welded along the trailing edge. This nominally inferior alloy was actually better in service due to easier quality control. Engines after 25 hours were not thrown away but overhauled.

Now, a change which would have made the Jumo 004 much more reliable would have been an improvement to the fuel control system known as the 'acceleration control valve' which had been developed and was entering production for service by April 1945.

Piston engines run of an air/fuel ratio of 14.2:1 which is 'stoichiometric' where there is excactly the amount of air required to burn all of the fuel. Jet engines need about 50:1. Any less an they tend to be in danger of destroying themselves from overheating.

The Jumo 004 system was fairly simple, too simple. Fuel was delivered by a shaft spool driven gear pump, this being a positive displacement gear pump that was chose to deliver a little more fuel than needed at full RPM at sea level. Subsequent to the pump the fuel went to a pressure relief valve that essentially regulated a constant pressure system.

The actual fuel flow would thus be controlled simply by how widely open a throttle valve was from its constant pressure supply. Open the valve, more fuel flows, air is heated more, expands more, spins up compressor until acceleration stops when it is at its new operating point.

A 2nd control device that became active above 6000 rpm was a centrifugal governor which was also set by the pilots throttle. This device adjusted the pressure to regulate engine RPM. In other words pilots throttle controlled engine RPM. Consider that at high altitude where the air was thin (20000ft 50%) less fuel was needed, the governor did all the work of compensating for altitude.

However make any adjustments to throttle and there was a risk of overheating the engine, burning out combustion chambers or shedding turbine blades as the fuel flow increased but the engine spool accelerated too slowly. You could also get a 'cold flame out' from overdosing the fuel and quenching the flame or a lean flameout as too little fuel was added.

The solution is to measure the actual compressor air mass flow rate and adjust the fuel pressure to suit, thus leaving little for the governor and throttle to control.

My favourite way is to put two pressure sensors at the change in area between the compressor outlet and the combustion inlet, the pressure change is related to mass flow rate and one can then use this to set fuel system pressure. Another is to use exhaust gas pressure and temperature (same as some throttle bottle carburettors) or to run engine inlet pressure and compressor pressure ratio through a 3 dimensional cam. The latter method is I think what Junkers used (or planned to use) if what Antony Kay writes in "German Jet Engines and Gas turbines" suggests to me.

This device was literally entering production and being delivered as the war ended. It would have made a huge difference in ease of handing for the pilots and engine reliability. Some ordinary Jumo 004B ran over 60 hours on the wing before being serviced (basically a turbine swap out). There were a great may sophisticated fuel control systems under development. Post war some German Jet engineers (eg Peter Kapsus from BMW) expressed regret at not having recognised the importance of precise control earlier.

The Arado 234A had been designed as a long range reconnaissance aircraft, something it did very well. The Ar 234B was optimised as a bomber but had a retractable undercarriage instead of a skid, with a widened body to compensate for the fuel space lost. This lost it 20 mph.

Me 262 could also carry out reconnaissance, over shorter ranges but higher speeds, possible an inferior camera.

The Ar 234 could use a BZA dive or shallow dive bombing bombsight or the Lotfe 7 linked to the autopilot like the Norden. The Me 262 had some issues with bombs slipstreaming and not detaching at high speeds as well as not being able to use either the BZA or Lotfe because of its cockpit vision.

The solution was either the TSA-2D toss bombing sight (detachment during the pull up) or blind bombing by EGON or Zyklops radio/radar control. TSA 2D worked very well and was being used by JG7. There was an earlier version of the TSA-2 simply known as the TSA-2 or 2A which was rejected in favour of the more advanced 2D which might have been ready by the Normandy invasion, but as we know, the engines were barely operationally usable.

The Ar 232C was a version of the Ar 234B with 4 engines. It was extremely fast, possibly the fastest aircraft of the war at sea level. A version of this had a two seat cockpit. This allowed for a bomb aimer navigator since using the pilot to both fly and aim the bombs was a little risky with allied aircraft in the air. It was mach, not thrust limited and had so much thrust it could carry 3000lbs of bombs at over 500mph.

I was not aware that the AR234 "C" became operational. Any more details about this?

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There is extensive information in the Creek and Smith book (monogram) "Arado 234 Blitz" including some 40 photographs of the Ar 234C Or "Caesar". Even the artwork on the cover is of a 234C.

The V19 was the first true prototype though there were experiments with 4 engine versions before then. A total of 4 production variants were delivered to Rechlin, the Luftwaffe test centre where they were flown. One was even shot down by a Mustang as its civilian test pilot practised slow flying.

Speed was about 548mph at 13000ft. The aircraft was Mach limited not thrust limited. One of the more interesting variants had a AWACS style rotadome/radom for a PPI display. Germans had settled on arrays of dielelectric rod aerials which produced this kind of lenticular antenna that was streamlined and suitable for U-boats, even aircraft and some ships.

Obviously the Ar 234B with a speed of only 465 mph was vulnerable to interception by up coming allied jets and even the faster piston fighters when carrying bombs, the Ar 234C was the answer to this dilemma, its relatively high excess thrust allowing it to carry external bombs without significant impact on speed.

Arado 234B = main production version with same engines but with
retractable undercarriage and semi recessed bomb bay, the fuselage was
widened 1.5 inches in order to maintain range. Speed dropped to 466
mph.

Arado 234C3 = Had 4 x BMW 003A-2 engines with 800kP thrust each and a
pressurized cockpit as well as a pair of forward firing 20mm guns.
About 4 actually entered troop testing. Speed was Mach limited to
between 528mph to 534mph by the onset of tail flutter at high altitude. There was also
a proposed armored high altitude fighter variant of this able to
intercept to 39,500ft and an improvised night fighter variant.

Feb 1945 order status; 1795 C3's

Arado 234C4 = recon of the above

Feb 1945 order status 330 C4's

Arado 234C5 = two seat cockpit for a navigator, bomb aimer, radio
opperator.

Arado 234P. Two seat night fighter version to come with a variety of
engine combos, cockpits under construction.

Arado 234 "Crescent Wing" a swept wing version with sweep and wing
thickness both progressively reducing at the tips to achieve constant
mach and good handling. The tail was also swept. Arado prototype Ar
234 V16 was to be mated to the new tail and wing which was being
fabricated until destroyed by allied troops in Feb 1945. The
prototypes V18, V26 and V30 were all allocated to this program.

Range reduced to around 650 miles with 3000lbs bombs but without drop tanks, for which 3
attachments were provided, though with much better takeoff, load,
climb, acceleration and speed. The aircraft was Mach limited rather
than thrust limited. Planed versions with the HeS 011 suffered no reduction
in range, in fact flew a little further.

However as the thrust of available engines grew a single pair of
engines could be used..

Basically these were the engines available
Jumo 004B-1 early production engine of 880kP thrust.
Jumo 004B-4 later, main production engine of 880kP thrust with hollow
air cooled turbine blades for greater reliabillity.
Jumo 004C used for some flight tests only 1025kP thrust.
Jumo 004D Just entering production as the war closed of 1050kP
thrust. Me 262 with this engine did 568mph in level flight.'
(some doubt in my mind about the status of the above but the 'accelerator control valve is well documented'

BMW 003A-2 800kg thrust used on Arado 234C
BMW 003E-2 800kg thrust used on Heinkel He 162 same as above with different
accessories gearbox for dorsal mounting. The 003E2 actually had a 30 second 900kg overload capability which gave the He 162 a 562mph burst of speed.
Might have been useful for take-off. Despite having a lower thrust than the 004 the 003 actually had a higher cruise thrust.
BMW 003C 900kP thrust using standard BMW 003A/E hot section but with a
new compressor for more thrust and efficiency for long range versions
of the Arado 234B/D
BMW 003D 1100kP thrust with new hot section and further improvied
compressor.

There is extensive information in the Creek and Smith book (monogram) "Arado 234 Blitz" including some 40 photographs of the Ar 234C Or "Caesar". Even the artwork on the cover is of a 234C.

The V19 was the first true prototype though there were experiments with 4 engine versions before then. A total of 4 production variants were delivered to Rechlin, the Luftwaffe test centre where they were flown. One was even shot down by a Mustang as its civilian test pilot practised slow flying.